Experimental and numerical evaluation of SnAgCu and SnPb solders using a MicroBGA under accelerated temperature cycling conditions

A comparative evaluation of the leading lead-free solder candidate (95.5Sn3.8Ag0.7Cu) and traditional tin-lead solder (63Sn37Pb) under thermal cycling conditions was carried out. A test vehicle consisting of four daisy chained 10×10 array 0.8mm pitch plastic micro ball grid arrays (microBGA) mounted on an 8-layer FR4 printed wiring board was designed. The board finish was organic solder preservative (OSP) for the lead-free devices and hot air solder levelled (HASL) in the case of the eutectic devices. An event detector was used to monitor the continuity of each daisy chain during accelerated temperature cycling, where the test vehicles were cycled with a ramp rate of approximately 3°C per minute from -40°C to 125°C, with 10-minute dwells and a total cycle time of 2 hours 10 minutes. Results to date plotted using a Weibull distribution indicate that the SnAgCu solder is more reliable under these conditions. Experiments were also carried out on large-scale lead-free solder specimens to determine the parameters required for the Anand viscoplasticity model. The Anand model was then implemented in finite element analysis using ANSYS®, where the submodelling technique was employed to determine the viscoplastic work per thermal cycle for each solder joint along the package diagonal. Schubert's fatigue life model was used to predict the number of cycles to failure of each joint, although it should be noted that the necessary model parameters for the may need to be calibrated. Results indicate that the joint under the die edge is likely to fail first and that the SnAgCu solder is more fatigue resistant. The numerical predictions underestimate the fatigue life in both cases.